1. Field of the Invention
The field of the invention is a composition of matter and a process for metal plating an aluminum surface.
2. Description of Related Art
Harrison et al. in an entitled article "Plated Aluminum Wheel Characterization," Metal Finishing, December 1994, pp. 11-16, notes that metal plating aluminum is one of the growing areas of decorative plating, especially aluminum automobile wheels. Although in the past plated aluminum wheels were a small after-market specialty item, this has become an original equipment manufacturer option and a special addition feature.
The major concerns in production of metal plated aluminum are the reliability of the plating process, appearance and cost.
In a typical sequence for applying metal coatings to aluminum, the substrate is polished and soak cleaned. The soak cleaner employed in the pretreatment of the aluminum surface removes finishing oils, grease and difficult-to-remove buffing compounds left on the surface of the aluminum from polishing.
After the soak clean, the aluminum is immersed in a mild caustic or alkaline etch solution operated at elevated temperatures since etch rate is more dependent on temperature than caustic concentration.
The mild alkaline etch removes the Beilby layer and roughens the surface. Employing aluminum-silicon alloys results in etching aluminum preferentially over the silicon, leaving coarse silicon crystals exposed on the surface.
An examination of the surface of the aluminum-silicon alloy shows large areas of exposed silicon interspersed within the aluminum matrix. The silicon particles vary in size, do not appear to be uniformly distributed throughout the casting, and are not uniformly distributed on the surface of the aluminum, but rather in discrete areas. Silicon crystals protrude from the surface, most of which are oriented perpendicular to the surface.
After the etch treatment, the substrate is then subjected to a desmut composition. Smaller, loosely adherent silicon particles (where a silicon containing alloy is employed), as well as intermetallic compounds, are most likely removed during the desmut step. The substrate is then rinsed, zincated, stripped with nitric acid, zincated again, and followed by a nickel strike coating. This in turn is followed by a bright copper plating, optional copper buffing, nickel plating and an optional high sulfur nickel to improve corrosion resistance. After these preparatory steps, a decorative chromium plate is applied.
As noted by Harrison et al., a film is left on the aluminum after the mild caustic etch that is removed by the desmut step, and is one of the most crucial steps in processing the aluminum substrate to ensure adequate adhesion of the subsequently applied metal coatings. The tenacity of this film varies with the composition of the aluminum, especially where an aluminum alloy is employed.
The desmut solution contains strong mineral acids, and when aluminum-silicon alloys are treated, fluoride ions. Both are selected to uniformly attack the aluminum surface, or the proportions varied to preferentially dissolve the silicon (e.g., high fluoride concentration) and/or the aluminum. The aluminum and exposed silicon particles are thereby rendered more active. Various combinations of additives, nitric, sulfuric, and phosphoric acids in combination with fluoride salts such as ammonium bifluoride or fluoroboric acid allow for adequate pretreatment of the aluminum to obtain good adhesion of subsequently applied metal coatings.
Aluminum wheels employed by the automotive industry are generally A-356 aluminum alloy castings. The A-356 alloy is generally chosen for aluminum wheel applications because of its ease of use in casting, high resistance to hot cracking, high fluidity, low shrinkage tendency and moderate ease of machinability.
The A-356 alloy is a hypoeutectic alloy consisting mainly of a two-phase microstructure. Iron is present to minimize sticking between the molds and casting. Magnesium and copper are added to impart strength to the alloy. Manganese is believed to improve the high temperature properties of the casting. The silicon in the alloy appears as very hard particles and imparts wear resistance. Most of the hypoeutectic aluminum-silicon alloy consists of a soft and ductile aluminum phase.
The nominal composition of A-356 aluminum alloys is as follows:
______________________________________ Element % by weight ______________________________________ Al 91.9 Si 7.0 Cu 0.2 Mg 0.3 Mn 0.1 Zn 0.1 Fe 0.2 Ti 0.2 ______________________________________
Treating aluminum alloys such as A-356 alloy in the foregoing manner leaves a heavy film on the aluminum after the mild caustic etch. This film or smut is a mixture of both aluminum oxides and alloying element oxides as well as exposed silicon in those alloys which contain silicon as an element.
The zincating materials generally consisted of CN zinc compositions that optionally contained nickel, and because of environmental reasons and the state-of-the-art cyanide treatment technology, manufacturers sought cyanide free systems.
Several cyanide free zincate compositions have been developed containing zinc and optionally nickel, copper, or iron and mixtures thereof; however, it was found in some instances that specific aluminum alloys, such as A-356 alloy, could not be pretreated satisfactorily in that a heterogeneous composition was formed on the surface of this alloy during the initial etch, sometimes referred to as segregation. This segregation in turn has an adverse affect on the adhesion of subsequently applied metal layers.
It was further found that aluminum-copper alloys such as 2024 alloy could not be etched uniformly either by alkaline or acid etchants due to its low solution potential. It was also found that this compromised the adhesion of subsequently applied metal coatings.
Ullman's Encyclopedia of Industrial Chemistry, Vol. A-1, p. 520 (1985), notes that the nature of the aluminum oxide surface and reactivity of aluminum after oxide removal makes electroplating more complicated. Additional factors include reactions between aluminum and the electroplating solutions, the galvanic reactions between aluminum and the plated metal, and the metallurgical structure of aluminum alloys that consists of solid solutions, constituents, dispersoids, and precipitates, each having a different reactivity.
As is apparent from the foregoing, metal plating of aluminum surfaces is a highly complex field.
It would therefore be an advantage to provide a process or composition for avoiding or minimizing the difficulties of smut formation, segregation, nonuniform etching and poor adhesion in the electrocoating of aluminum substrates in a process that utilizes cyanide-free zinc compositions.
Accordingly, the present invention is directed to a composition of matter and a process that substantially obviates one or more of these and other problems due to limitations and disadvantages of the related art.